The present invention relates generally to electrical circuitry which provides overload protection to associated circuitry components and more particularly to a circuit for overload protection of a plurality of cascode coupled switching transistors coupled between terminals of a d.c. voltage source.
In solid state switching circuits one or more power transistors are generally used as load switches. For reliable operation of the switching circuit, these transistors require protection from excessive power dissipation during fault of transient conditions. In circuitry having a plurality of transistors used as load switches coupled between terminals of a d.c. voltage source, the necessity for overload protection of the switching transistors is increased since failure of one transistor in likelihood would cause inoperability of the entire series circuit. The terms "fault" and "transient conditions" are here used herein in a general sense. The former denotes, for example, a condition in which associated circuitry supplies an improper voltage to the switching transistors or the output load is inappropriately shorted or opencircuited. Transient conditions (or transients) include a myriad of short time duration conditions which differ from the intended steady-state or normal operation of the circuit. A simple example is a switching circuit applying a sudden voltage or current change to an inductive or capacitive load. The associated circuitry experiences a momentarily large current or voltage. The switching circuitry may be designed to accommodate these conditions; however, such overdesign of circuit components typically results in a very inefficient circuit under normal operating conditions. As described in the applicant's earlier invention, U.S. Pat. No. 4,158,866, issued on June 19, 1979, assigned to the assignee of the present invention, a protection circuit which is responsive to transient or other overvoltages developed across the collector/emitter electrodes of a switching transistor may be used to prevent damage to the device.
Present technology power transistors for practical high power switching applications are generally limited to switching voltages below about .+-.600 volts. Series stacking the emitter collector paths of plural first thru n.sup.th switching transistors, as taught by U.S. Pat. No. 4,270,163 permits relatively high power switching application of voltage levels substantially exceeding the maximum safe switching voltage rating for individual switching transistors. Although the circuitry taught in the above referred application overcomes the problems associated with matching static and dynamic parameters of stacked switching transistors, such circuitry is vulnerable to the previously described transient and fault conditions, which may degrade or destroy the switching transistor and associated circuitry. The present invention provides a protection circuit for switching circuits having a plurality of stacked switching transistors. The inventive circuitry senses the voltage developed between the collector and emitter electrodes of each of the switching transistors. When this voltage exceeds a predetermined value for a conducting transistor, the protection circuit responds by removing the base drive from the transistor, rendering it substantially non-conductive.
The art has generally recognized the requirement for overload protection of switching devices. These references typically teach monitoring either the current flowing through the associated circuitry to be protected or the input voltage thereto. In U.S. Pat. No. 3,582,713, for example, excessive load current causes an increased voltage drop across a base to emitter path of a bypass transistor, which shunts current away from associated circuitry during overload conditions. Associated circuitry monitors the input voltage using a zener diode to bypass current away from the protected circuitry during overvoltage conditions. A second example of overvoltage protection is found in U.S. Pat. No. 3,571,608, where a circuit input voltage produces a base drive signal to turn on an associated bypass transistor, which turns off the switching transistor.